Planarer Mikrobrenner in Mikrosystemtechnik
Final Report Abstract
Focus of the micro-burner project was its application as a flame ionization detector (FID). The FID is the most popular and widely used detector for the analysis of trace levels of organic compounds. Reduction of FID gas consumption by miniaturization enhances its portability. In addition, MEMS technology allows for a low cost, disposable device. The flame burns in the silicon plane of an almost completely encapsulating glass-silicon-glass sandwich. It provides optimum thermal shielding and prevents contamination from the environment. Electrodes for ion current measurement can be deposited on the inner glass surface above and below the flame. Main problem of the planar design was the finite resistance of the borosilicate glass at elevated temperatures. As a result, a leakage through the glass caused a high background current, which impaired an accurate measurement of low ion currents. The problem was solved by integrating an additional electrode, the so-called guard electrode. When well-positioned in between the two other electrodes, it intercepts the leakage current and bypasses it past the current detector. The design of this guard electrode was optimized in several steps. The optimum design consists of a chromium thin-film guard electrode underneath the platinum thin-film measurement electrode at the inner surface of the bottom glass substrate. Both electrodes are electrically separated by a sputtered silicon oxide layer. Whereas without guard electrode a leakage current of several nA was recorded, with guard electrode only 100 pA or less was measured. For comparison, a sample containing 1 % methane gives a signal of up to 4.4 nA. First designs are based on a single-nozzle micro-burner for premixed oxyhydrogen flames. However, it was found that an opposed-nozzle micro-burner for counter-current diffusion flames shows better FID performance at even smaller gas consumption. The main advantage can be attributed to a stagnation point with zero flow velocity in between the opposed nozzles. As a result, heat loss by forced convection is small and the residence time of fuel (complete combustion) and sample (increased ion yield) is large. FEM simulations of flow and convection show that the flame location is determined by the stagnation point. Because it is at a certain distance from the nozzles, not only heat loss by convection, but also by conduction through the nozzles is minimized. The simulations also predict an increasing degree of confinement of the flammable mixture with decreasing distance between the nozzles and decreasing opening area of the oxygen nozzle. Simulation results are confirmed by experimental flame observations (brightness scales with the degree of mixture confinement) and FID sensitivity measurements (sensitivity scales with the degree of mixture confinement). Accordingly, largest absolute sensitivity of 13.7 mC/gC is obtained with the smallest nozzles and the smallest distance in between (1 mm) at 7.5 ml/min hydrogen and 9.4 ml/min oxygen. For comparison, typical FID sensitivity of 15 mC/gC is obtained at typical gas flows of 30 ml/min hydrogen and 300 ml/min synthetic air. In both cases, optimum performance is achieved under fuel lean conditions. Whereas maximum absolute sensitivity is measured at 2.0 ml/min sample gas flow and with the smallest nozzles, maximum relative sensitivity of 0.583 pA/ppm is obtained at 8.0 ml/min sample gas flow and with the largest nozzles. In this case smaller nozzles are disadvantageous, because flames split in half at increased sample gas flows. The unfavorable surface area-to-volume ratio of such dumbbell-shaped flames results in a considerable decrease in sensitivity. Therefore, depending on the application, either small (maximum absolute sensitivity) or large (maximum relative sensitivity) nozzles are preferred. Peak-topeak noise only depends on the polarization voltage and measures 1 pA at 50 V. As a result, the absolute and relative MDLs are 1.46·10^-10 gC/s and 3.43 ppm methane, respectively. The cc-µFID can be used as a detector in µGC (gas chromatography) without the need for additional make-up gas. It is used to determine the difference in cc-µFID sensitivity and response factors of hydrogen (normal mode) and oxygen premixed samples. It is concluded that both sensitivity and the homogeneity of response factors to methane, ethane, propane, and butane are reduced considerably, when the sample is premixed with the oxygen instead of with the hydrogen. This result supports the classical theory of H-atom induced decomposition of organic compounds to single-carbon fragments.
Publications
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Planar micro flame ionization detector, Proceedings of the 10th Annual Workshop on Semiconductor Advances for Future Electronics and Sensors (SAFE `07), Nov. 29-30, 2007, Veldhoven, The Netherlands
Kuipers, W.J. and Müller, J.
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Planar micro flame ionization detector, Proceedings of the 18th Workshop on MicroMechanics Europe (MME `07), September 16-18, 2007, Guimarães, Portugal
Kuipers, W.J. and Müller J.
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Planarer Mikroflammenionisationsdetektor, Proceedings of the 2nd Mikrosystemtechnik-Kongress, Oct. 15-17, 2007, Dresden, Germany
Kuipers, W.J. and Müller, J.
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A planar micro-flame ionization detector with an integrated guard electrode, J. Micromech. Microeng. 18 (2008)
Kuipers, W.J. and Müller, J.
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Planar micro flame ionization detector with leak current reducing guard electrode, Proceedings of the 12th Int. Meeting on Chemical Sensors (IMCS `08), July 12-14, 2008, Columbus, USA
Kuipers, W.J. and Müller J.
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Influence of micro-burner geometry on flame stability, Proceedings of the 3rd Mikrosystemtechnik-Kongress, Oct. 10-12, 2009, Berlin, Germany
Kuipers, W.J., et al.
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Performance of a planar micro flame ionization detector, Proceedings of the 3rd Mikrosystemtechnik-Kongress, Oct. 12-14, 2009, Berlin, Germany
Kuipers, W.J. and Müller J.
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Planar micro flame ionization detector with minimized leak current, Proceedings of the 15th International Conference on Solid-State Sensors, Actuators and Microsystems (Transducers `09), June 21-25, 2009, Denver, CO, USA
Kuipers, W.J. and Müller, J.
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Total hydrocarbon analysis with a planar micro flame ionization detector, Proceedings of the 8th Annual IEEE Conference on Sensors (Sensors `09), Oct. 25-28, 2009, Christchurch, New Zealand
Kuipers, W.J. and Müller, J.
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Positive size effect on the sensitivity of a counter-current micro flame ionization detector, Proceedings of the 9th Annual IEEE Conf. on Sensors (Sensors `10), Nov. 1-3, 2010, Waikoloa, HI, USA
Kuipers, W.J. and Müller, J.
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Sensitivity of a planar micro-flame ionization detector, Talanta 82 (2010) 1674-1679
Kuipers, W.J. and Müller, J.
